436 VETERINARY PHYSIOLOGY 



2. Why is there no Pulse in the Veins? — Their walls have 

 a certain amount of elasticity, but, instead of there being a 

 resistance to the outflow of blood from the veins into the 

 heart, this is favoured by the suction action of the thorax 

 in inspiration. Hence, even if an intermittent inflow were 

 well marked, the absence of resistance to outflow would in 

 itself prevent the development of a venous pulse. But the 

 inflow is not intermittent. The arteries are so overfilled 

 that just as much blood j^cisses into the veins between the 

 beats as during the beats of the heart. Hence the most 

 important factor in causing a pulse, an intermittent inflow, 

 is absent. 



With no intermittent inflow, and with no resistance to 

 outflow, the development of a pulse is impossible. 



In certain abnormal conditions, where, from the extreme 

 dilatation of the arterioles, the inflow into the veins is very 

 free, and where the outflow from the part of the body is not 

 so free, a local venous pulse may develop. 



A special pulse in the great veins near the heart is 

 considered on p. 444. 



3. Characters of the Pulse Wave. — If a finger be placed on 

 the carotid artery and another upon the radial artery, it will 

 be felt that the artery near the heart expands (pulses) before 

 that further from the heart {Practical Physiology). The 

 pulse develops first in the arteries near the heart and passes 

 outwards towards the periphery. The reason for this is 

 obvious. The arteries are always overfilled with blood. 

 The ventricle drives its contents into the overfilled aorta, 

 and, to accommodate this, the aortic wall expands. But, 

 since the aorta communicates with the other arteries, this 

 increased pressure passes outwards along them, expanding 

 their wall as it goes. 



It greatly simplifies the study of the pulse to regard it 

 in this light, and to study it just as we should study a wave 

 at sea. 



1. Velocity. — To determine how fast a wave is travelling, 

 the time may be ascertained which it takes to pass from 

 one point to another at a known distance from the first. 

 So with the pulse wave : two points on an artery at a known 



